1. Field of the Invention
The present invention relates to a polypeptide as a novel fusion protein, a polynucleotide encoding the polypeptide, a vector comprising the polynucleotide, a transformed cell comprising the vector, a method for detecting the fusion protein or polynucleotide, a method for screening a therapeutic agent for cancer, and a therapeutic agent for cancer.
2. Background Art
(Carcinogenesis and Gene)
Several cancer-related genes have been known so far. In particular, tyrosine kinase genes, which encode important enzymes directly regulating cell growth, have been known to be activated even by substitution or deletion in amino acid sequences and thereby bring about carcinogenesis (Non-Patent Document 1).
For example, BCR-ABL fusion genes are found in most of patients with chronic myeloid leukemia. Proteins produced by this abnormal gene cause the abnormal growth of leukemia cells and simultaneously tend to inhibit blood cell apoptosis, leading to the onset of chronic myeloid leukemia (Non-Patent Document 24). Imatinib mesylate, an inhibitor of ABL tyrosine kinase, is effective for the treatment of this disease. Alternatively, TEL-JAK2 fusion proteins have been reported to be observed in acute lymphoblastic leukemia, while NPM-ALK fusion genes encoding NPM fused with ALK tyrosine kinase are observed in more than half of the cases of anaplastic large cell lymphoma (ALCL) and the activation of ALK kinase has been shown to be important for tumor cell growth by NPM-ALK (Non-Patent Documents 25 and 14).
(Lung Cancer and Oncogene)
In 2004, Paez et al. (Non-Patent Document 2) and Lynch et al. (Non-Patent Document 3) have shown that epidermal growth factor receptor (EGFR) genes having sequence abnormalities are expressed in some lung cancer cells. They have also reported that gefitinib (trademark: Iressa), a kinase activity inhibitor of EGFR, is therapeutically effective for patients having these EGFR mutations. Subsequent analyses have demonstrated that EGFR mutations are frequently observed in Asians, nonsmokers, and female patients with lung cancer, and that gefitinib is significantly effective for some of these cases (Non-Patent Documents 4 and 5).
Regarding the involvement of tumor suppressor genes, it has previously been reported that the inactivation of TP53 gene and Rb pathway occurs in lung cancer with a high frequency (Non-Patent Document 6). By contrast, regarding an active type of oncogene that strongly positively induces the cell growth of lung cancer, only KRAS1 gene activation in some cases has been reported (Non-Patent Document 7). The presence of a new type of abnormal kinase has been reported to be found in approximately 10% of lung cancer cases, and this report, however, has made no reference to specific molecules (Non-Patent Document 36).
In 2000, EML4 (echinoderm microtubule-associated protein like protein 4) (Non-Patent Document 26) has been reported as a cytoplasmic protein with a molecular weight of 120,000, which is highly expressed in the M phase of the cell cycle (Non-Patent Document 8). A human EML4 gene encodes a polypeptide with 981 amino acids and has 23 exons. This gene has been mapped to chromosome 2. The EML4 protein has a basic region at the amino terminus, as with other members of the EML family, and further has carboxyl-terminal WD domains. The physiological functions of EML4 have been little known. However, according to a recent report, EML4 participates in microtubule formation (Non-Patent Document 9).
On the other hand, ALK (Anaplastic Lymphoma Kinase) (Non-Patent Document 27) is receptor tyrosine kinase. This protein has a transmembrane domain in the central part and has a carboxyl-terminal tyrosine kinase region and an amino-terminal extracellular domain (Non-Patent Document 28). The ALK gene, which has 30 exons encoding a polypeptide with 1620 amino acids, has been mapped to chromosome 2. This ALK gene has been thought, from the site or timing of its expression, to participate in the development or functions of the nervous system (Non-Patent Document 10). Loren et al. have reported from the homolog analysis of Drosophila ALK that ALK participates in muscle differentiation (Non-Patent Document 11). However, no abnormality has been observed in ALK knockout mice, and its distinct physiological functions still remain to be elucidated (Non-Patent Document 12).
Full-length ALK expression has been reported so far in some cancer cells of ectodermal origin, such as neuroblastoma, glioblastoma, breast cancer, and melanoma (the full-length ALK expression has not been observed in cancer cells of endodermal and mesodermal origins) (Non-Patent Document 13). Full-length ALK is expressed in many neuroblastoma cell lines. However, the autophosphorylation of ALK is not observed in these neuroblastoma cell lines. Moreover, ALK expression has been reported, from the cohort analysis of neuroblastoma patients, to be weakly associated with canceration. It has been suggested that ALK expression in neuroblastoma may reflect its expression in normal neural differentiation, rather than its association with canceration (Non-Patent Document 10). On the other hand, in reported cases, ligands such as pleiotrophin and midkine as well as the gene amplification of ALK itself increase the autophosphorylation of ALK and mobilize intracellular signals. It has also been reported that ALK may contribute to cancer cell growth (Non-Patent Document 12).
In some cases of human malignant lymphoma and inflammatory myofibroblastic tumor, the ALK gene has been reported to be fused with other genes (NPM, CLTCL, TFG, CARS, SEC31L1, etc.) as a result of chromosomal translocation or inversion and thereby form a fusion type of tyrosine kinase (Non-Patent Documents 14 to 19 and 29 to 33). Moreover, a method for identifying a protein as a fusion partner for ALK using ALK antibodies has been reported (Non-Patent Document 35). On the other hand, a fusion gene of EML4 and ALK has not been reported. The intracellular localization of these ALK fusion proteins depends on a fusion partner molecule for ALK, and the ALK fusion proteins have been known to exist in cytoplasm, nucleus, and the like. Since most partner molecules have a complex formation domain, the fusion protein itself has been thought to form a complex. This complex formation has been considered to cause loss of control of the tyrosine kinase activity of ALK and induce carcinogenesis with abnormally activated intracellular signals (Non-Patent Document 10). Indeed, it has been reported that the use of ALK shRNA or ALK kinase-inhibiting compound for lymphoma cells expressing ALK fusion proteins can induce cell growth inhibition and cell death. Therefore, it has been suggested that the ALK fusion protein may serve as a therapeutic target for lymphoma and inflammatory myofibroblastic tumor (Non-Patent Documents 20 to 22). It has also been suggested that ALK may serve as a therapeutic target for other cancers whose growth involves ALK as described above (Non-Patent Documents 21 to 22).
Various low-molecular-weight compounds having an inhibitory activity against ALK have been reported so far. Marzec et al. have reported that WHI-P131 and WHI-P154 (both, EMD Biosciences), which have originally been utilized as JAK3 tyrosine kinase-inhibiting substances, inhibit the activity of NPM-ALK (Non-Patent Document 22). Another group has developed their own low-molecular-weight ALK-inhibiting substance and has demonstrated that this inhibitor induces the cell death of NPM-ALK-expressing lymphoma cell lines (Non-Patent Document 21). In addition, plural low-molecular-weight compounds having an inhibitory activity against ALK have been reported so far (Non-Patent Documents 23 and 34 and Patent Documents 1 to 4).
[Patent Document 1] Pamphlet of WO 2005/097765
[Patent Document 2] Pamphlet of WO 2005/009389
[Patent Document 3] Pamphlet of WO 2005/016894
[Patent Document 4] Pamphlet of WO 2004/080980
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[Non-Patent Document 26] GenBank accession Number: NM 019063
[Non-Patent Document 27] GenBank accession Number: AB209477
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[Non-Patent Document 35] PNAS 2006 103, 7402-7407
An object of the present invention is to elucidate a polynucleotide as a novel oncogene and thereby provide a method and kit for detecting the polynucleotide, a method for screening a therapeutic agent for cancer, a method for treating cancer, and a therapeutic agent for cancer.